> Would the WHERE statements I have limiting what kind of items being returned mess it up?
You can *NOT* use WHERE with the *dependent* table! (That is, the right side table in a LEFT join. It would be the left side table in a RIGHT join, of course.)
If you do, you CONVERT the LEFT JOIN (or RIGHT JOIN) to an INNER JOIN!!!!
Any conditions that you want to put on the dependent table *MUST* be part of the ON condition and *NOT* be in the WHERE!
Let's say you currently have
WHERE items.size > 73
So move that to the ON, thus:
IFNULL( GROUP_CONCAT(items.id), '--none--' ) AS itemslist
FROM a LEFT JOIN items
ON items.owner = a.username AND items.size > 73
GROUP BY a.username
Next time, show us the entire query and we'll be able to help you better.